Photoimageable, aqueous acid soluble polyimide polymers

ABSTRACT

A photoimageable, aqueous acid soluble polyimide polymer comprising an anhydride, including a substituted benzophenone nucleus, a diamine reacted with the anhydride to form a photosensitive polymer intermediate, and at least 60 Mole % of solubilizing amine reacted with the photosensitive polymer intermediate to form the photoimageable, aqueous acid soluble polyimide polymer. An emulsion for electrophoretic deposition of a coating of a photoimageable, aqueous acid soluble polyimide polymer comprises a dispersed phase, including the photoimageable aqueous acid soluble polyimide polymer, dissolved in an organic solvent and a dispersion phase including a coalescence promoter and water. The emulsion may be applied, by electrophoretic deposition, to a conductive structure to provide a photoimageable coating on the conductive structure. After exposing the coating to a pattern of radiation for photocrosslinking exposed parts of the photoimageable aqueous acid soluble polyimide polymer, an aqueous acid developer solution removes unexposed photoimageable aqueous acid soluble polyimide polymer to reveal a crosslinked polyimide polymer image of the radiation pattern.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is a divisional application claiming priorityfrom U.S. application Ser. No. 10/038,280, filed Jan. 3, 2002, nowallowed, which is a divisional application of U.S. application Ser. No.09/547,390, filed Apr. 11, 2000, now issued as U.S. Pat. No. 6,379,865B1.

FIELD OF THE INVENTION

[0002] This invention relates to coating formulations and a method,useful in microelectronics applications, for isolating and protectingfine-pitch, electrically conducting circuit interconnects, and relatedstructures. More particularly the invention provides coating materialsfor application to conductive elements using an electrophoreticdeposition technique. The coatings provide protective, high resistivity,low dielectric constant, negative image bearing layers after exposure toradiation patterns of suitable wavelength, followed by development withmild aqueous acid solutions.

BACKGROUND TO THE INVENTION

[0003] Modern society relies upon the trouble-free conveniences providedby electrical and electronic devices. Since the earliest recognitionthat useful devices could be developed by combining electrical circuits,circuit combinations have become more complex, and the resulting devicesmore sophisticated in their capabilities. Effective circuit performancerelies upon electrical current isolation within a particular circuitwith no possibility of current leakage into a neighboring circuit. Anyunintended current transfer between circuits of a multi-circuit,multi-function electrical device will ultimately cause an inconvenientmalfunction of the device.

[0004] Isolation or insulation of circuits from each other represents anincreasing challenge with the continuing emphasis on more complexprinted circuit designs and increased functionality for electricaldevices, especially miniature electronic devices. Progress in electricaldevice design has caused a transition from the interconnection ofdiscrete electrical components, using pre-insulated wiring structures,to interconnection, with modern printed circuits, using conductivetraces only microns wide. Protection and isolation of such narrowtraces, from each other, demands materials that may be precisely placedover the elongate current carrying traces while leaving tiny contactpoints exposed for electrical connection to other circuits that formpart of a particular device. For a significant period of time it waspossible to essentially cover the printed circuit with a protectivecoating, leaving voids in the coating corresponding to the needed pointsof contact. More recently, however, the introduction of flexible printedcircuits and multi-layer printed circuits has led to the need forcoatings and processes capable of high precision in protective coverformation and placement. High precision techniques provide a cover-layerwith essentially just sufficient insulation to protect a conductivetrace without straying into other portions of a printed circuitsubstrate. Such coatings tend to be very thin and subject to attack by,e.g. solvents, moisture, or other potentially damaging environments. Forthis reason, precision coating of printed circuits must provide bothinsulative and environmental protection for electrical conductors.

[0005] A variety of coating methods exists for applying coatings,covercoats and the like as protective, insulating coatings to printedcircuit patterns. The term covercoat refers to a dielectric coating,over the printed circuit basestock, applied after the conductive circuitpattern has been etched. The covercoat serves to protect the copperconductors from moisture, contamination and damage. Conventional coatingmethods include screen printing and application of continuous layers bymethods such as knife coating, spin coating, extrusion coating, dipcoating, curtain coating, and spray coating. Application of continuouscoatings covers not only the leads but also the area in between theleads. This condition has several disadvantages when found inintricately structured printed circuits. For example, differences inexpansion coefficients between a continuous cover-coat and a flexibleprinted circuit substrate may introduce stresses that cause the circuitto adopt an inconvenient curl-set. Segmentation of a cover-coat, intoseparate coated areas, is less likely to be subject to this condition.

[0006] Selective deposition processes, such as electrophoreticdeposition, also known as “e-coat,” may achieve coating separation andprecise positioning (details of this process may be found in the“Handbook of Electropainting Technology” by W. Machu, ElectrochemicalPublication Limited, 1978). Application of electrophoretic depositiontechniques began at least three decades ago for painting automobiles andappliances. Electrophoretic deposition involves precise distribution ofa layer of charged droplets over a conducting surface that represents anelectrode of an electrolytic cell operating under direct currentpotential. Charged droplets migrate towards an oppositely chargedelectrode to be deposited thereon. Droplet deposition and layerformation may occur at either an anode or a cathode. Preferably thedroplets are positively charged for deposition on a cathodic surface.Cathodic coatings do not suffer the oxidative corrosive processesassociated with anodic deposition. Also, electrophoretic deposition ofwater-based compositions produces essentially void free andsubstantially non-polluting coatings.

[0007] Compared to conventional coating processes, such as screenprinting, electrophoretic deposition selectively places a protectivelayer only on conductive portions of the printed circuit. Use ofelectrophoretic deposition should produce individually encapsulatedconductors, whereas conventional techniques coat the entire printedcircuit. Selective deposition also offers other advantages, such as theproduction of lighter weight circuits which is important for hard diskdrive (HDD) flexible circuits applications.

[0008] The use of electrophoretic deposition is known for coatingprinted circuits with photoresists. U.S. Pat. Nos. U.S. 4,845,012; U.S.5,055,164; U.S. 5,607,818; U.S. 5,384,229; U.S. 5,959,859; and U.S.5,439,774 contain reference to the technique. Other United States Pat.Nos. U.S. 4,592,816 and U.S. 5,181,984 describe epoxy/acrylatecompositions for electrophoretic deposition of solder mask/covercoatsystems. Photoresist and solder mask materials are typicallyphotosensitive and developable to a patterned polymer, covering selected(imaged) portions of the printed circuit. This provides evidence ofphotoimageable coatings, formed by electrophoretic deposition.Additionally, U.S. Pat. No. U.S. 4,832,808 teaches electrophoreticdeposition of coatings of piperazine-containing polyimides. However,such coatings possess neither photosensitivity nor solubilization inaqueous acid developers.

[0009] The effective use of electrophoretically deposited,photoimageable coatings may depend upon the image resolution attainablewith such systems. Printed circuits of increasing density require theuse of photoresists of increasing image resolution. Image resolutiondepends upon radiation scattering within photosensitive layers and thevariation of image characteristics, i.e. resolution, related todevelopers and development processes.

[0010] Polyimide-containing formulations provide potentially usefulmaterials for photoimageable coatings produced by electrophoreticdeposition. They also have the thermal and dielectric propertiessuitable for protecting and insulating electrical current carryingconductors. Image development of polyimide coatings, after exposure toan image pattern, may involve non-aqueous, solvent-based developers oraqueous-based developers. The use of solvent-based development systemsapplies to photoimageable polyimides that may use a benzophenone moietyas a built-in photo-crosslinker. U.S. Pat. Nos. U.S. 4,629,685; U.S.4,656,116; U.S. 4,841,233; U.S. 4,914,182; U.S. 4,925,912; U.S.5,501,941; U.S. 5,504,830; U.S. 5,532,110; and U.S. 5,599,655; andEuropean Patent No. EP 0456463 A2 provide evidence of autosensitizedpolyimides. As indicated previously, these materials need organicsolvents for image development. High volume use of solvent developers,in production operations, may cause environmental problems associatedwith solvent pollution and disposal. Aqueous developers provide a moreenvironmentally friendly alternative to organic solvent developers. Somealkaline aqueous developers contain tetramethylammonium hydroxide as anagent for image development of photoimageable polyimides derived fromeither polyamic acid or phenolic derivatives. These precursors tend toproduce polyimides having residual reactivity, leading to copper oxideformation, when deposited on copper, along with related corrosion ofmetallic copper that could result in poor coated film properties.

[0011] Considering the disadvantages of previously discussed,solvent-based and alkaline aqueous image developers and the benefits ofselective coating deposition processes, there is a need forelectrophoretically deposited, photoimageable polyimide coatings,soluble in non-polluting, preferably aqeous image developers.

SUMMARY OF THE INVENTION

[0012] The present invention provides photoimageable polyimide coatingsapplied from emulsion or solution formulations using electrophoreticdeposition techniques. Such coatings function as image recordingmaterials through exposure to a pattern of suitable radiation. An image,formed in a coating according to the present invention, may be revealedusing an acidified aqueous developer. An intended use of thesephotoimageable polyimides is the precise placement of protective,electrically insulating coatings over conductive parts of a printedcircuit pattern, followed by imagewise exposure and development toremove the coating from those parts of the circuit that provide pointsof connection to other circuits or electrical devices. Acidified aqueousdevelopers offer advantages over previously discussed solvent andaqueous alkaline developers by preventing problems of copper corrosionand copper oxide formation. The use of photoimageable, aqueous aciddevelopable polyimides distinguishes coating materials, according to thepresent invention, from materials using less desirable types of imagedeveloper.

[0013] More particularly the invention provides a photoimageable,aqueous acid soluble polyimide polymer comprising an anhydride,including a substituted benzophenone nucleus, a diamine reacted with theanhydride to form a photosensitive polymer intermediate, and at least 60Mole % of solubilizing amine reacted with the photosensitive polymerintermediate to form the photoimageable, aqueous acid soluble polyimidepolymer. An emulsion for electrophoretic deposition of a coating of aphotoimageable, aqueous acid soluble polyimide polymer comprises adispersed phase, including the photoimageable aqueous acid solublepolyimide polymer, dissolved in an organic solvent and a dispersionphase including a coalescence promoter and water. The emulsion may beapplied, by electrophoretic deposition, to a conductive structure toprovide a photoimageable coating on the conductive structure. A methodfor imaging a photoimageable aqueous acid soluble polyimide polymerapplied to a conductive structure, used for connecting electrical orelectronic components, comprises the steps of, providing a conductivestructure used for connecting electrical and electronic components, andapplying a coating to the conductive structure using an electrophoreticcoating technique. The coating comprises an anhydride including asubstituted benzophenone nucleus, a diamine reacted with the anhydrideto form a photosensitive polymer intermediate, and at least 60 Mole % ofa solubilizing amine reacted with the photosensitive polymerintermediate. Thereafter, exposing the coating to a pattern of radiationfor photocrosslinking exposed parts of the photoimageable aqueous acidsoluble polyimide polymer, and applying an aqueous acid developersolution to remove unexposed photoimageable aqueous acid solublepolyimide polymer to reveal a crosslinked polyimide polymer image of theradiation pattern.

[0014] Electrophoretic deposition techniques allow relatively preciseplacement of material on charged surfaces included in an electrolyticcell, operated by direct current. The charged surfaces could includesuitably connected printed circuits to induce material placement onindividual metal traces of the circuitry. Using electrophoreticdeposition techniques, deposition of material occurs predominantly onconductive surfaces. This facilitates the coating of unsupported leadsand relatively inaccessible portions of a printed circuit such asconductive traces disposed within the structure of a multilayer circuit.Traditional coating methods do not provide desirable protection for suchfeatures. In addition, precision coating via electrophoretic depositiontechniques uses less material than traditional coating methods therebyproviding beneficial cost savings and waste reduction. The selectiveplacing of electrophoretically deposited films provides an addedadvantage, for coating flexible printed circuits, compared to blanketinglayers produced with conventional coating methods. Regardless ofdifferences in coefficient of thermal expansion, selectively depositedcoatings cannot exert a force to distort the general shape of theflexible substrate material. Flexible circuits, coated usingelectrophoretic deposition, are lighter and less likely to exhibitcure-stress-induced curl after processing. Lower circuit weight isimportant for certain applications, such as interconnects for hard diskdrives.

Definitions

[0015] For clarification, the following definitions provide the meaningof terms that may be used throughout this specification.

[0016] The term “covercoat” refers to a dielectric coating, over thebasestock, applied after the conductive pattern has been etched. Thebasestock may be a conventional printed circuit substrate, includingflexible polyimide sheet, used as a support for etched metal patterns,particularly those formed by etching copper.

[0017] The term “current density” means the amount of current flowingthrough a substrate, per unit area, perpendicular to the direction ofcurrent flow.

[0018] The term “e-coat” is synonymous with electrophoretic depositionand may refer herein to a coating, and technique for electrophoreticallydepositing such a coating.

[0019] The terms “emulsion” and “solution” are used somewhatinterchangeably to refer to polyimide containing fluids that may beunderstood as conventional emulsions except when suspended particlesbecome so small that the liquid is essentially clear with little or noevidence of turbidity, i.e. its visual appearance is that of a solution.When the “emulsion” used for electrophoretic deposition appears topossess solution-like properties, it is considered as a solution and isso described herein.

[0020] The term “unsupported lead” means a conductive trace or lead thatspans a void in a substrate or extends over the edge of a substrate andthereby exists in an unsupported condition.

[0021] The term “mole % amine” as used herein is based upon the originalpopulation of anhydride groups before reaction with a diamine to form aphotosensitive polyimide moiety.

[0022] For example, 60 Mole % of solubilizing amine represents an amountequivalent to 60% of the anhydride groups available in the anhydridestarting material.

[0023] The “polymer intermediate” refers to a reaction product, of atleast two monomers, that has the capability for further reaction withother selected reactants. Anhydrides reacting with diamines, asdescribed herein, produce polymer intermediates for further reactionwith solubilizing amines.

[0024] The term “solubilizing amine” refers to materials containingamine functionality that may react with polymer intermediates toincrease polymer solubility in solutions of aqueous acid.

[0025] The term “aqueous acid soluble polymer” refers to a polymer thatis at least partially soluble in aqueous acid solutions.

[0026] The term “aqueous acid developable polymer” refers to aphotoimageable, aqueous acid soluble polymer crosslinked by exposure tosuitable radiation so that crosslinked material no longer dissolves indilute aqueous acid. This allows dissolution of unexposed material toleave an insoluble pattern of crosslinked material corresponding to thepattern of radiation used for exposure.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027]FIG. 1 provides a perspective end view of a section of printedcircuit wherein conductive traces have a conventional protectivecoating.

[0028]FIG. 2 provides a perspective end view of a section of printedcircuit wherein conductive traces have a protective coating according tothe present invention.

[0029]FIG. 3 provides a perspective end view of a section of printedcircuit wherein conductive traces have a protective coating according tothe present invention and the conductive traces include an unsupportedlead.

DETAILED DESCRIPTION OF THE INVENTION

[0030] The present invention provides modified photoimageable polyimidematerials, having solubility in mildly acidic aqueous solutions. Thesematerials are suitable as electrically insulating protective layers fordelicate, fragile conductive traces of the type produced by etchingcopper layers to form printed circuits.

[0031] Referring now to the drawings, wherein like parts have likeidentifying numerals throughout the several views, FIG. 1 depicts asection 10 from a printed circuit. The section includes a substrate 12supporting/a plurality of conductive traces 14. A coating 16 protectsthe conductive traces 14 from environmental contaminants and, at thesame time, electrically insulates the traces 14 from one another. Thecoating 16 covers, as a covercoat, the surfaces of both the conductivetraces 14 and the substrate 12 between the traces 14. A covercoat ofthis type results from the use of conventional coating techniques, suchas dipping, extrusion coating or spray coating.

[0032] The section of printed circuit 20 of FIG. 2 differs from that ofFIG. 1 by illustrating a photoimageable coating 18 which coversindividual conductive traces 14 with an essentially uniform layer ofprotective, insulating material according to the present invention. Theuse of an electrophoretic deposition coating technique limits thephotoimageable coating 18 to the conductive traces 14 leaving thesubstrate 12 between the traces 14 substantially free of coatingmaterial.

[0033]FIG. 3 shows a section 20 of printed circuit that includes anunsupported lead 17 as an extension of one of the conductive traces 14that projects beyond the edge of the substrate 12. Such unsupportedleads 17, although fragile, are common in complex printed circuits usedfor interconnection of electronic devices. Protection and insulation ofunsupported leads, is difficult to accomplish using conventional coatingtechniques. The use of electrophoretic deposition techniques simplifiesthe task, by producing a uniform layer over the entire surface of theunsupported lead portion 17 of one of the conductive traces 14.

[0034] An aqueous acid developable photoimageable polyimide, accordingto the present invention, may be prepared by reacting a suitableanhydride molecule, containing a substituted benzophenone nucleus, withan aromatic diamine to form a photosensitive polymer intermediate whichbecomes aqueous acid soluble upon reaction with a solubilizing amine.

[0035] Suitable anhydrides are usually substituted benzophenonedianhydrides and related structures including:benzophenone-tetracarboxylic dianhydride; anthraquinone-tetracarboxylicdianhydride; fluorenone-tetracarboxylic dianhydride;thioxanthone-tetracarboxylic dianhydride. Mixed anhydrides may be usedwith optional anhydrides providing up to 25% of the total anhydridecontent. Optional anhydrides include: biphenyl-tetracarboxylicdianhydride; 3,3′diphenylsulfone-tetracarboxylic dianhydride;4,4′oxydiphthalic anhydride;2,2′-bis(3,4-dicarboxyphenyl)hexafluoropropane; and bis-dicarboxyphenylsulfide.

[0036] Suitable aromatic diamines include: tetramethylphenylene diamine;4,4′methylene-bis(2-methylaniline); 4,4′methylene-bis(2-ethylaniline);4,4′methylene-bis(2-dimethylaniline);4,4′methylene-bis(2-diethylaniline); dimethylphenylenediamine;trimethylphenylenediamine; 2,4-dimethyl-1,5-phenylenediamine; and2,4,5-trimethyl-1,3-phenylenediamine. Diamine mixtures may contain up to25% of optional diamines including: 1,4-bis(4-aminophenoxy)benzene;4,4′-oxydianiline; 2,2′-bis(4-aminophenyl) hexafluoropropane;4,4′-methylenedianiline and terminal difunctional amino-siloxanes.

[0037] The polyimide materials become image forming upon exposure topatterns of radiation, usually ultraviolet radiation. The solubility ofthese polyimides in dilute solutions of weak acid, before exposure,provides a way to develop the image patterns. Amine functionalityincorporated into the polyimide structure improves the solubility of theresulting polymers in aqueous solutions of weak acids. The aminefunctionality should be present at a molar concentration preferably ofabout 60 Mole %, or greater, to offer acid developability in 0.2-0.5%aqueous acetic acid. Preferably the soluble polymer is a reactionproduct containing at least about 60 Mole % to about 70 Mole % of aneffective amine.

[0038] Aqueous acid solubility of polymers according to the presentinvention may also be a function of the molecular weight of the productof the reaction of a polymer intermediate and a solubilizing amine.Photoimageable aqueous acid soluble polymers exist in the molecularweight range from about 20,000 to about 300,000 and preferably fromabout 50,000 to about 150,000 with a molecular weight distribution(Mw/Mn) from about 1.1 to 3.5, preferably from about 1.5 to 3.0.

[0039] Solubilizing amines suitable for imparting polyimide solubilityin aqueous acid solutions include: 1 -methylpiperazine,1,-(N,N-dimethylamino)-3 -(N′-methylamino) propane(N,N,N′-trimethyl-ethylenediamine), 2-(dimethylamino)morpholine,3-(dimethylamino)piperidine, 1-methyl-4-(2-methylamino-ethyl)piperazine,bis-2-dimethylaminoethyl(N,N,N′,N′,-tetramethyldiethylenetriamine),N,N-dimethylethylenediamine, 1-hydroxymethylpiperazine,1-hydroxyethylpiperazine, N,N-dimethylamino-ethanol, and3-dimethylaminopropylamine, with 1-methylpiperazine being particularlypreferred.

[0040] Aqueous acid solutions, suitable for developing the polymericcompositions, may be selected from acetic acid, ethoxyacetic acid,propionic acid, butyric acid, lactic acid, glycolic acid, formic acidand succinic acid and mixtures thereof.

[0041] The invention also includes a method for applying coatings ofphotoimageable polyimide compositions, preferably by electrophoreticdeposition of material onto conductive surfaces. Electrophoreticdeposition occurs by migration of charged particles or droplets presentin the electrolyte of an electrochemical cell. Application of apotential difference to the cell causes charged particles to migrate tothe electrode of opposite charge where charge neutralization, on theconductive surface, causes material to deposit and coat the electrode.Electrophoretic coating, according to the present invention, requiresthe application of direct current and coating deposition at thenegatively charged electrode, or cathode. Specifically, the polymer isdeposited from an emulsion or solution bath containing some amount ofsolvent soluble polymer dispersed in an aqueous dispersion phase. Asdiscussed herein, electrophoretically depositable emulsions or solutionsinclude a polyimide composition dispersed in an appropriate medium orcarrier. The carrier comprises water and a coalescing solvent. Withcurrent flowing between electrodes immersed in the solution or emulsionbath, particles or droplets of the dispersed phase begin preferentialmigration towards the electrode of opposite charge.

[0042] Control of current density and voltage is important forelectrophoretic deposition coatings to promote adhesion and prevent theformation of porous coatings. An appropriate current density is in therange of about 0.6 mA/cm² to about 5.0 mA/cm². Preferably the currentdensity range is from about 0.8 mA/cm² to about 1.5 mA/cm². The appliedpotential difference or voltage bias may be between about 10 volts andabout 100 volts. Preferably, the applied voltage is between about 20volts and about 50 volts. For this range of voltages a suitable powersupply is a Hewlett Packard 6633A System DC Power Supply with a maximumvoltage of 50 volts and a maximum current of 2 amps. As used herein theterm “current density” means the amount of current flowing through asubstrate, per unit area, perpendicular to the direction of currentflow.

[0043] Coated film thickness and quality depends upon the e-coatformulation, voltage, current density, and the duration of the appliedcurrent. The electrophoretic deposition process should be limited to afew minutes. Times from about 1 minute to about 3 minutes areacceptable, but longer coating times may result in films possiblycontaminated with acid residues sufficient to cause corrosion at theelectrode surface. Such attack could damage a copper printed circuittrace functioning as an electrode for electrophoretic deposition. Other,factors that affect electrophoretically deposited coating thicknessinclude solid content of the emulsion, emulsion particle size,conductivity of the emulsion and pH.

[0044] Polyimide coating formulations, according to the presentinvention, include solutions but preferably comprise oil in wateremulsions having an average dispersed phase particle size preferablyfrom about 0.002 μm to about 20 μm in diameter and even more preferablyless than about 2 μm in diameter. Such emulsions may be prepared bydissolving the polymer in an organic solvent such asN-methylpyrrolidone, γ-butyrolactone, dimethylformamide or the like,adding an acid and a coalescing solvent and adding water. Emulsionpreparation involves the use of a high-speed blender with acidneutralization for improved stability. Suitable neutralizing acidsinclude lactic or acetic acid. Emulsions of the present inventionexhibit shelf life and effective performance for periods of at leasteight months and usually more than one year without agitation.

[0045] Electrophoretically deposited coatings of these emulsions containcoalescence promoters which cause coating coalescence immediately afterthe electrophoretic deposition process. The process also providesexcellent edge coverage that could result in uniform coverage ofconnecting leads, both supported and unsupported, used as currentcarriers in printed circuits and related structures. Coalescencepromoters include n-butyl-cellosolve, propylene glycol monomethyl ether,propylene glycol ethyl ether acetate, propylene glycol methyl ether,propylene glycol n-butyl ether, propylene glycol n-propyl ether,propylene glycol phenyl ether, dipropylene glycol, propylene glycol,propylene carbonate, propylene glycol ethyl ether, propylene glycolmethyl ether acetate, ethylene glycol monomethyl ether, butylene glycol,diethylene glycol, diethylene glycol ethyl ether, ethoxyethanol, ethoxyethanol acetate, ethylene glycol, triethylene glycol, ethylene glycoldiacetate, ethylene glycol propyl ether, methoxy ethanol acetate,methoxy ethanol, phenoxy ethanol, n-butanol, di(ethylene glycol) butylether, 2-ethyl hexanol, acetophenone, toluene, propylene glycol methylether acetate, and selected mixtures thereof, with n-butyl-cellosolvebeing preferred.

[0046] After drying an electrophoretically deposited coating atapproximately 85° C., an image may be formed by exposing the coating toan pattern of ultraviolet radiation. The photoimaging process occurs viacrosslinking exposed areas of the polyimide layer using a photo-maskbetween the coating and a broadband ultraviolet lamp. Photoimaged,crosslinked polyimide no longer dissolves in dilute aqueous acidsolution. Image patterns, corresponding to the photo-mask, may bedeveloped by dipping the imaged coating in a solution containing fromabout 0.1% to about 0.5% of acetic acid in water. This removes theunexposed, non-crosslinked polyimide that is still soluble in aqueousacid. After acid development, followed by rinsing withtetramethylammonium hydroxide solution and deionized water, thedeveloped image, corresponding to the pattern of ultraviolet radiation,may be fixed by curing at least about 300° C., preferably at least about350° C., in a nitrogen filled oven. The heating rate and dwell time atthe image curing temperature require relatively careful control. Apositive nitrogen pressure prevents oxidation of the imaged film duringcuring. The use of a slow rate of heating, to the final curingtemperature, allows volatile products to escape before the polyimidecures fully. A controlled heating rate also prevents foaming and filmdelamination. Generally cured polyimide films, as covercoats on flexiblecircuits, exhibit good adhesion (ASTM D3359), hardness of about 3Hpencil hardness (ASTM D3363), without cracking, during bending at aradius of 0.3 mm, and less curl compared to conventional epoxy acrylatecovercoats.

[0047] Aqueous acetic acid development provided images with lines of ±25microns (±1 mil) resolution. Electrophoretically deposited coatings fromabout 1.0 μm to about 15.0 μm provide this level of image definition.Preliminary work with spray development suggests a reduction in theamount of time to develop films thicker than 15.0 μm. Spray developmentoffers advantages over dip development. The use of aqueous aciddevelopers is convenient and environmentally beneficial compared tosolvent-based developers which require disposal, after use, incompliance with environmental regulations. Photoimageable polyimides,according to the present invention, may be developed, after exposure,with a 2:1 volume ratio of N-methyl pyrrolidone:methyl alcohol, but thismixture is much less environmentally compatible than the preferredaqueous acid developer.

[0048] Electrophoretic deposition of photoimageable polyimides onconductive surfaces should provide a relatively precise approach forcovering and protecting fragile leads, i.e. unsupported leads, used forelectrical interconnection in high density printed circuits includingflexible circuits. Photoimageable polyimide materials, applied in thisway, have potential application as barrier coatings to provide abrasionresistance and electrically insulating protective layers for productapplications in areas such as integrated circuit packaging (ICP), inkjet printers, hard disk drives, medical and biomedical equipment andautomotive applications.

Experimental Preparation of Photosensitive Polyimides

[0049] Dianhydride and diamine monomers were added to a nitrogen-filled500 ml flask. A quantity of 1-methyl-2-pyrrolidinone (NMP) was added tothe flask with stirring to produce a solution of monomers in NMP. Theresulting viscous solution was maintained under nitrogen with stirringfor sixteen hours. After cooling the solution to 0° C., using anice-bath around the flask, a solution containing 1,3dicyclohexylcarbodiimide in NMP was added dropwise. During the addition,the color of the solution adopted a dark reddish color, typical ofpolyisoimide polymers. Upon completion of this addition, the flask andcontents were allowed to warm up to room temperature and stirringcontinued for a further extended period of approximately fifteen hours.Upon addition of 1—methyl piperazine, there was evidence of an exothermand, within about fifteen minutes, the solution changed color, from darkred to light pink.

[0050] The contents of the flask were stirred under nitrogen for afurther six hours then filtered into approximately three liters of analcohol and water mixture containing 1 part of methyl alcohol to 2 partsof water. A pink colored solid formed as a precipitate in the alcoholand water solution. This was isolated by filtration and dried undervacuum at room temperature.

[0051] This method was used for preparation of Examples 1 and 2 shown inTable 1.

EXAMPLES

[0052] TABLE 1 Photoimageable Polyimide Compositions Example 1 Example 2BTDA 10.58 g (32.8 mmol) 33.22 g (103.0 mmol) MBDMA 13.13 g (51.6 mmol)TMPDA  5.39 g (32.8 mmol)  8.45 g (51.4 mmol) NMP 150 ml 300 ml DDC 17.5 g (84.8 mmol)  51.0 g (248 mmol) in 100 ml NMP in 150 ml NMPPiperazine  6.7 ml (60 mmol)  20.6 ml (185 mmol) Mn 92,000 258,000 Mw/Mn1.83 1.60

Emulsion Preparation

[0053] A quantity of photoimageable polyimide, dissolved in1-methyl-2-pyrrolidinone (NMP) was stirred at 1000 rpm, using aDispermat® FE Laboratory Dissolver from VMA-GETZMANN. After dropwiseaddition of lactic acid and butyl cellosolve, the stirring speed wasincreased to 5000 rpm. Water was added dropwise to the rapidly stirringsolution until the emulsion phase inverted from water in solvent tosolvent in water. An additional quantity of water was added and theemulsion was kept stirring for 2-3 minutes. Finally, the fluidcomposition was filtered through a 1.0 μm filter into a 4 oz. wide-mouthjar for storage.

Cathodic Electrophoretic Deposition of Photoimageable Coatings on Copper

[0054] Emulsion samples were placed in the reservoir of anelectrochemical cell having a cathode, in the form of a copper layer ona flexible polyimide substrate, and a platinized anode separated fromthe cathode by a fixed distance. A Hewlett Packard, 6633A DC powersupply was connected to the electrodes. The maximum voltage bias andcurrent density settings were selected. Polyimide deposition, under theselected conditions, continued for about two minutes before the cathodewas removed from the emulsion to be rinsed with deionized (DI) water,from a spray bottle, then immersed in dilute, 0.1N tetramethylammoniumhydroxide (TMAH) solution for about two seconds. After a final rinsewith deionized water, the sample was dried in a conventional ovencontrolled at 85° C. for 15 minutes. The deposition process produced asmooth polymer film on the surface of the cathode. TABLE 2 CoatingFormulations and Conditions for Electrophoretic Deposition CoatingCoating Formulation 1 Formulation 2 Polyimide Example 1-3.5 g Example2-4.66 g NMP 14 g 18.6 g 85% solution of Lactic acid 0.92 ml 0.62 mlButyl cellosolve 10 ml 13.8 ml Water 65 ml 78.4 ml pH 4.46 5.80Conductivity (μS) 1330 780 Electrode separation 3.0 cm 4.0 cm Voltagebias 50 V 20 V Current density 1 mA/cm² 1 mA/cm² Time of deposition 2minutes 2 minutes Drying time at 85° C. 15 minutes 10 minutes

Image Formation and Development

[0055] A copper substrate, coated with a 6 μm thick layer of the polymerof Example 1, was exposed, through a mask, to a wide spectrumultraviolet radiation source (available from Hybrid TechnologyGroup—Model #LS66-1OX-220/254 UV Lamp). The intensity of exposure wasapproximately 30 mW/cm² for about 40 seconds (1200 mJ/cm²).

[0056] The exposed layer was developed in a 0.1% solution of aqueousacetic acid for about 30-40 seconds followed by rinsing with water,tetramethylammonium hydroxide (TMAH) solution, and a second water rinsebefore drying and baking the developed coating. The baking or thermalcuring of imaged samples took place in a nitrogen filled oven accordingto a thermal profile that included 2 hours at 260° C., then 0.5 hour at300° C. and a final bake step of 5 minutes at 350° C. Image resolutionof about 25 μm (1 mil) was observed following this process.

[0057] Using essentially the same process, under the followingconditions, a coating of Example 2 gave an image coating 6 μm thick.

[0058] Imaging conditions: UV exposed with a mask at 1000 mJ/cm² doseand developed with 0.2% acetic acid for about 60 seconds secondsfollowed by rinsing with water, tetramethylammonium hydroxide (TMAH)solution, and a second water rinse before drying and baking thedeveloped coating.

[0059] Final cure: 3° C./min. ramp to 240° C. (2 hrs); 1° C./min ramp to300° C. (1 hr.) and 1° C./min. to 350° C. (15 min.).

[0060] The solubility of modified polyimides, in aqueous acid solutions,may be varied by adjusting the concentration of the reactive amine.Investigation of the relationship between amine content and acidsolubility using 1-methyl piperazine indicates the need for a highpiperazine content (>60%) for sufficient solubility in aqueous acidsolution. Modified polyimides containing less than 60% 1-methylpiperazine have less solubility in aqueous acids making such solutionsless effective as image developers. TABLE 3 Acid solubility data ofPolyimides Modified with 1-methyl piperazine* Mol. % Solubility in 0.5%Acetic Solubility in 40% piperazine group Acid at room temperature.Acetic Acid at 65° C. 35% No No 55% No No 60% Yes Yes 67% Yes Yes

[0061] Studies of emulsions, prepared with the polyimide of Example 1,show changes in pH and conductivity with increasing addition of butylcellosolve. As shown in Table 4, conductivity decreases and pH appearsto increase slightly with butyl cellosolve addition. Conductivity wasmeasured using a Coming PS-17 conductivity meter. The pH was measuredusing a Corning pH-30 Sensor calibrated with Ricca Chemical Company pH 4and 7 buffer solutions. TABLE 4 Variation of pH and Conductivity ofEmulsions. Sample-1 Sample-2 Sample-3 Sample-4 Polyimide of Example 13.5 g 3.5 g 3.5 g 3.5 g Lactic Acid (85%) 0.92 mL 0.92 mL 0.92 mL 0.92mL NMP  14 g  14 g  14 g  14 g Water 70 mL 67.5 mL 65 mL 65 mL ButylCellosolve 0  2.5 mL  5 mL 10 mL pH 4.30 4.34 4.39 4.46 Conductivity(μS) 1860 1720 1550 1330 Coating Appearance A B B C

[0062] Emulsions shown in Table 4 appear almost transparent suggestingan emulsion particle size less than the wavelength of visible light(0.4-0.8 μm). The particle size of emulsions of this invention may bebelow 0.1 μm (100 nm).

[0063] A photosensitive, aqueous acid soluble polyimide polymer andrelated coatings have been described according to the present invention.It will be appreciated by those of skill in the art that, in light ofthe present disclosure, changes may be made to the embodiments disclosedherein without departing from the spirit and scope of the invention.

What is claimed is:
 1. An emulsion for electrophoretic deposition of acoating of a photoimageable, aqueous acid soluble polyimide polymer,said emulsion comprising: a dispersed phase including saidphotoimageable, aqueous acid soluble polyimide polymer dissolved in anorganic solvent; and a dispersion phase including at least onecoalescence promoter and water.
 2. An emulsion according to claim 1wherein said organic solvent is selected from the group consisting ofN-methylpyrrolidone, y-butyrolactone and dimethylformamide.
 3. Anemulsion according to claim 1 wherein said coalescence promoter isselected from the group consisting of n-butyl-cellosolve, propyleneglycol monomethyl ether, propylene glycol ethyl ether acetate, propyleneglycol methyl ether, propylene glycol n-butyl ether, propylene glycoln-propyl ether, propylene glycol phenyl ether, dipropylene glycol,propylene glycol, propylene carbonate, propylene glycol ethyl ether,propylene glycol methyl ether acetate, ethylene glycol monomethyl ether,butylene glycol, diethylene glycol, diethylene glycol ethyl ether,ethoxyethanol, ethoxy ethanol acetate, ethylene glycol, triethyleneglycol, ethylene glycol diacetate, ethylene glycol propyl ether, methoxyethanol acetate, methoxy ethanol, phenoxy ethanol, n-butanol,di(ethylene glycol) butyl ether, and 2-ethyl hexanol.